A plunger lift is typically an apparatus that can be used to increase the productivity of oil and gas wells. In the early stages of a well's life, liquid loading may not be a problem. When production rates are high, well liquids are typically carried out of the well tubing by high velocity gas. As a well declines and production decreases, a critical velocity may be reached wherein heavier liquids may not make it to the surface. Rather, the heavier liquids may start to fall back to the bottom of the well. This liquid drop can exert back pressure on the formation, which “loads up” the well. As a result, the gas being produced by the formation can no longer carry the liquid being produced to the surface. As gas flow rate and pressures decline in a well, lifting efficiency can decline substantially.
Liquid drop may occur for two reasons. First, as liquid comes in contact with the wall of the production string of tubing, friction slows the velocity of the liquid. Some of the liquid may adhere to the tubing wall, creating a film of liquid on the tubing wall which does not reach the surface. Second, as the liquid velocity continues to slow, the gas phase may no longer be able to support liquid in either a slug form or a droplet form. Along with the liquid film on the sides of the tubing, a slug or droplet(s) may begin to fall back to the bottom of the well. In a very aggravated situation there will be liquid accumulated in the bottom of the well. The produced gas must bubble through the liquid at the bottom of the well and then flow to the surface. However, as gas advances through the accumulated liquid, the gas may proceed at a low velocity. Thus, little liquid, if any, may be carried to the surface by the gas, resulting in only a small amount of gas being produced at the surface. A plunger lift can act to remove the accumulated liquid.
A plunger system is a method of unloading gas in high ratio hydrocarbon wells without interrupting production. A plunger lift system utilizes gas present within the well as a system driver. Generally, wells making no gas are not plunger lift candidates.
A plunger lift system works by cycling a well open and closed. During operation, a plunger typically travels to the bottom of a well where loading fluid may be picked up or lifted by the plunger and brought to the surface, thus removing all liquids in the tubing. The plunger can also keep the tubing free of paraffin, salt or scale build-up. During the open time, a plunger interfaces a liquid slug and gas. The gas below the plunger will push both the plunger and the liquid on top of the plunger to the surface. As liquid is removed from the tubing bore, an otherwise impeded volume of gas can begin to flow from a producing well.
In U.S. Pat. Pub. No. US 2004/0226713 A1 dated Nov. 18, 2004, Townsend describes a plunger with an elongate body having two ends, a sleeve overlying the body and having a first and second end and an interior bore and being shorter in length than the elongate body. The plunger has a circumferential seal on the exterior surface of the sleeve to provide a barrier to the passage of gas or fluids during closure. The elongate body is a solid body and flow passes directly into the sleeve. The flow passes through the sleeve between the elongate body and the sleeve when the bypass function is open during plunger descent to the well bottom. The outer diameter of the elongate body and the inner diameter of the sleeve are not constant throughout, allowing for radial movement between the two pieces at one end.